Although a powder comprising a carbon material has been conventionally used in a negative electrode active material for a lithium secondary battery, the carbon material has a low theoretical capacity of 372 mAh/g and there is a limit on further increase of the capacity thereof. In contrast, in recent years, the application of a metallic material of which the theoretical capacity is higher than that of a carbon material, such as Sn, Al or Si, has been examined or put into practical use. Particularly, Si is a promising material, as it has a theoretical capacity of more than 4000 mAh/g. Using these metallic materials which substitute carbon as negative electrode active materials for a lithium secondary battery achieves a high capacity, but the problem is its short cycle life.
There have been proposed many methods of performing improvement against the problem by adding various elements to Si to make not a pure Si powder but a Si alloy powder to obtain a fine structure. For example, in Japanese Patent Laid-Open Publication No. 2001-297757 (Patent Literature 1), the amount of an element such as Co to be eutectic or the larger amount thereof to be hypereutectic is added and the resultant is solidified at a cooling rate of 100° C./s or more to obtain an alloy powder with the minor axis particle diameter of a Si phase of 5 μm or less. Cycle life is improved by using such a Si alloy powder having a fine Si phase. That is, the effect of suppressing the change of the volume of a fine SI phase during absorption/desorption of Li is obtained by generating a silicide which neither absorbs nor desorbs Li.